The present invention relates to a safety ignition interlock switch device for a vehicle transmission.
The need for safety ignition interlock devices in tractors and other vehicles is well known. Such a device prevents the engine of the vehicle from being started when the transmission is engaged and avoids the risk of accidents being caused by the vehicle jumping into movement unexpectedly when the engine is started. Some tractors and other vehicles have selector mechanisms with two selector rods which select different sets of gears. Such mechanisms are known in synchronized and crash gearboxes and also in transmissions with a plurality of gear groups or ranges, for example, the transmission described in commonly assigned U.S. patent application, Ser. No. 302,327, filed Sept. 15, 1981, now U.S. Pat. No. 4,438,657. Both selector rods have neutral positions and, although a mechanical interlock ensures that only one rod can be out of neutral at any time, it remains necessary to check that both rods are in neutral to provide the safety interlock for the ignition circuit.
Safety ignition interlock devices are known in commercially available tractors in which the switch elment is a plunger which slides in a bore in the bracket perpendicular to the parallel selector rods. The plunger is a grounded contact which, in its first position, presses against a fixed contact mounted in insulated fashion through an adjacent part of the transmission housing. Although this device performs the required function, it is not suitable for use with all transmissions. Vehicle transmissions are invariably compactly designed units with a high density of components. The space available for fitting a safety switch device is restricted. Moreover, the fixed part of the device, such as the fixed contact, has to be at an accessible location where it can be installed in the housing and reached for maintenance. In fact, it is found that in some transmissions, the required position for the plunger of the known device interferes with a component of the transmission itself and it is therefore impossible to install the known device in such a transmission.